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Engineering

ThermodynamicsChapterTwo

TheEssenceofFirstLawofThermodynamicsJilinUniversityThermalenergyengineeringdepartment2023/2/281Applicationsofdifferentenergyequationexpressions.Theuniversalpurposeofthefirstlawofthermodynamics.Definitionofsystemenergy,thermodynamicenergy,expansionwork,flowwork,shaftwork,technicalwork,andtherelationshipamongthem.Theskilltoanalyzethesystemenergybalanceforthesystem.TasksJilinUniversityThermalenergyengineeringdepartment2023/2/282KeynotesTheessenceoffirstlawofthermodynamics.Distinguishthedifferencebetweenthermodynamicenergyandsystemenergy.Energyequationsforopensystemandclosedsystem.Relationshipsamongthetechnicalwork,shaftwork,flowworkandexpansionwork.Applicationsofsteadystatesteadyflowenergyequation.JilinUniversityThermalenergyengineeringdepartment2023/2/283§2.1DefinitionoffirstlawThefirstlawofthermodynamics,knownastheconservationofenergyprinciple,providesasoundbasisforstudyingtherelationshipsamongthevariousformsofenergyandenergyinteractions.Energycanbeneithercreatednordestroyedduringaprocess;itcanonlychangeforms.Thefirstlawstatesthatenergyisconserved:Thechangeintheinternalenergyisequaltotheamountaddedbyheatingminustheamountlostbydoingworkontheenvironment.JilinUniversityThermalenergyengineeringdepartment2023/2/284§2.1DefinitionoffirstlawThefirstlawcanbestatedmathematicallyas:

dU=δQ-δWwheredU

isasmallincreaseintheinternalenergyofthesystem,δQisasmallamountofheataddedtothesystem,andδWisasmallamountofworkdonebythesystem.JilinUniversityThermalenergyengineeringdepartment2023/2/285The"OverbalancedWheel".Itwasthoughtthatthemetalballsontherightsidewouldturnthewheelbecauseofthelongerleverarm,butsincetheleftsidehadmoreballsthantherightside,thetorquewasbalancedandtheperpetualmovementcouldnotbeachieved.§2.1DefinitionoffirstlawJilinUniversityThermalenergyengineeringdepartmentAperpetualmotionmachineofthefirstkindproducesworkwithouttheinputofenergy.Itthusviolatesthefirstlawofthermodynamics.2023/2/286TotalenergyenteringthesystemTotalenergy

leaving

thesystemChangeinthetotalenergyofthesystem=-§2.1DefinitionoffirstlawJilinUniversityThermalenergyengineeringdepartment2023/2/287§2-2Closedsystemenergyequation

Specificthermodynamicenergyu/(J/kg)：TotalsystemenergyE/J：

Expressionofspecificthermodynamicenergy：kineticpotential1.Thermaldynamicenergyofsystem:

Systemenergy

E:

E＝Ek＋Ep＋UJilinUniversityThermalenergyengineeringdepartment2023/2/288Heatandwork：QandWForthermodynamicprocess1-2：Fortheinfinitesimalprocess：§2-2Closedsystemenergyequation

JilinUniversityThermalenergyengineeringdepartment2023/2/289Forunitmass：Definitionofsign：plusforabsorbheat,minusforreleaseheat;Positivevalueforgasexpression,andnegativevalueforgascompression.ItappliestoQuasi-staticStateProcessandNon-quasi-staticStateProcess.§2-2Closedsystemenergyequation

JilinUniversityThermalenergyengineeringdepartment2023/2/2810forQuasi-staticStateProcesswithoutdissipationso：Forunitmass：§2-2Closedsystemenergyequation

JilinUniversityThermalenergyengineeringdepartment2023/2/2811§2-3Opensystemenergyequation

PhysicalmodelJilinUniversityThermalenergyengineeringdepartment2023/2/2812energychangedinsystemfordτ

：therefore：Thisequationcalledmassequationorcontinuityequation,itistherelationbetweeninputandoutputmassoftheopensystem.

Energyexchange：①heat：②shaftwork：attheinput,flowwork：§2-3Opensystemenergyequation

JilinUniversityThermalenergyengineeringdepartment2023/2/2813FlowworkAttheexit，theflowworkforpushingouttheinfinitesimalmass：

③thenetflowworkoutofthesystemis：

④thedifferencebetweeninandout:JilinUniversityThermalenergyengineeringdepartment2023/2/2814energyequationofopensystem

：forand（2-6a）andFlowworkJilinUniversityThermalenergyengineeringdepartment2023/2/2815so：FlowworkJilinUniversityThermalenergyengineeringdepartment2023/2/2816§2-4Steadystatesteadyflowenergyequation

Processfeature：Substituteinto（2-6a），thenifJilinUniversityThermalenergyengineeringdepartment2023/2/2817ifsoItiscalledenthalpy,itisalsoastateparameter：theenthalpyisalsoonlydependentonforanidealgas.

§2-4Steadystatesteadyflowenergyequation

JilinUniversityThermalenergyengineeringdepartment2023/2/2818forso：Thatis§2-4Steadystatesteadyflowenergyequation

JilinUniversityThermalenergyengineeringdepartment2023/2/2819§2-5Shaftworkfor：then：JilinUniversityThermalenergyengineeringdepartment2023/2/2820Technicalworkwt—themechanicalenergycanbeapplieddirectlySubstitutetheexpressionofshaftworkintotheequation：Omitthekineticandpotentialenergy,then§2-5ShaftworkJilinUniversityThermalenergyengineeringdepartment2023/2/28212-6ExamplesofSSSF1.RadiatorandCondenser2.TurbineorCompressorfeature：feature：so：so：JilinUniversityThermalenergyengineeringdepartment2023/2/28223.NozzleandDiffuser4.ThrottlingValveso：feature：feature：so：JilinUniversityThermalenergyengineeringdepartment2023/2/2823Thefirstlaw

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